1. Field of the Invention
The present invention relates to an electro-optical device such as a liquid crystal device that uses a liquid crystal device substrate, and a projection display device that employs the same. More particularly, this invention relates to the structure of a type of liquid crystal device and electro-optical device in which reset potentials are applied to respective data lines before image signals are supplied to the data lines.
2. Description of Related Art
Of the liquid crystal device substrates employed for a liquid crystal device, the one having a built-in driving circuit has a pixel section 11 in which pixel areas 40 are defined by a plurality of scanning lines 20 and a plurality of data lines 30 arranged in a matrix pattern on a substrate 10 as shown in the block diagram of FIG. 15. Each pixel area 40 has a pixel switching TFT 50 (thin film transistor) connected to the scanning line 20 and the data line 30, a liquid crystal cell, and a holding capacitor formed between itself and a capacitor line 29. Formed in the area outside (the area around) the pixel section 11 on the substrate 10 are a data side driving circuit section 60 for supplying image signals to each of the plurality of data lines 30 and a scanning side driving circuit section 70 for supplying scan signals to each of the plurality of scanning lines 20. Of these driving circuit sections 60 and 70, the data side driving circuit section 60 has an X-shift register 610 and a sampling circuit 620 equipped with a TFT functioning as an analog switch, so that image signals are supplied to each of the data lines 20 via an image signal line 630.
In a liquid crystal device that employs a liquid crystal device substrate 1 configured as described above, for example, in order to implement a so-called inversion driving system in which image signals invert the polarity of data signals for each line according to the potential of a counter electrode or invert the polarity of the voltage applied to liquid crystal, the image signals supplied to the data lines 30 (a source electrode of the TFT 50) are written to the liquid crystal cell via the TFT 50 while the polarity thereof is inverted for each horizontal scanning period as illustrated in FIG. 16A; hence, the potential of the pixel electrode of the pixel switching TFT changes as shown in FIG. 16B. This means that the polarity of the image signals is inverted for each horizontal scanning period; therefore, the potential of the pixel electrode greatly changes and the charge and discharge from the data lines 30 to the image signal line 620 is accordingly repeated. Such charge and discharge seldom affects the quality of display if the display conforms to the NTSC standard because the sampling rate is relatively low; however, if the display based on HDTV or double speed NTSC is carried out, the charge and discharge leads to the generation of noise because the sampling rate is higher.
To avoid the problem set forth above, a configuration has been proposed wherein a reset driving circuit 80, which is equipped with two series of reset signal lines 81 and 82 for applying reset potentials to the respective data lines 30 before supplying the image signals to the data lines 30 by utilizing a horizontal retrace line zone or the like and a reset potential supply ON/OFF switching circuit 83, are provided in the area outside the pixel section 11 as shown in FIG. 15, so that the charge and discharge from the data lines 30 is almost completed beforehand by the reset potentials. In the liquid crystal device substrate 1, which has this configuration, a reset potential having a predetermined polarity is applied from the reset signal lines 81 and 82 just before image signals are supplied to the data lines 30 as shown in FIG. 16C. Thus, since the charge and discharge from the data lines 30 can be mostly completed before image signals are supplied to the data lines 30, the potential of the pixel electrode changes less with time as illustrated in FIG. 16D, making it possible to restrain the charge and discharge from the data lines 30. This in turn makes it possible to prevent the fluctuation of the potential at the image signal line 630, so that the generation of noises in display can be controlled.
When bonding the liquid crystal device substrate 1 to a counter substrate (not shown) with a predetermined cell gap secured between the substrates, a sealant containing a cell gap material is applied considerably inward from the outer periphery of the liquid crystal device substrate 1 because a counter substrate 5 is smaller than the liquid crystal device substrate 1 as shown in FIG. 15 and FIG. 17, and the liquid crystal device substrate 1 and the counter substrate 5 are bonded by a sealing layer 90 composed of the sealant, the inner area thereof being provided as a liquid crystal sealing area 12. In an example shown in FIG. 17, the sealing layer 90 is formed slightly outside the reset signal lines 81 and 82 in the area outside the pixel section 11. In the area where the sealing layer 90 is formed, the process for forming the scanning lines 20 or the like is directly used to form many dummy patterns 15 in parallel to match the geometric shapes of the data lines on the opposite side, so that these parts are seemingly flat in the entire panel, and the sealant is applied thereto.
In the conventional configuration wherein the reset potentials are applied to the respective data lines 30 before the image signals are supplied to the data lines 30 so as to perform the charge and discharge from the data lines 30 by the reset potentials, since the pixels arranged laterally are reset at the same time; therefore, signals (electric charges) circulate to other data lines 30 via the reset signal lines 81 and 82 depending on the display pattern in the preceding frame. Such signal circulation appears in terms of a lateral cross talk in display, presenting a problem of deterioration in display quality. These kinds of problems could be prevented by setting time constants of the reset signal lines 81 and 82 sufficiently higher than time constants of the data lines 30; hitherto, however, there has been only one method available by which the width of the data lines 30 is increased to make the time constants of the data lines 30 side relatively smaller, and the method is not adequate to securely prevent the signals from circulating.
Accordingly, the present invention has been made to solve the problem described above and it is an object of the invention to provide a configuration that increases the time constant itself of a reset signal line to prevent signals from circulating from the data lines via the reset signal line, thereby to improve the quality of display in a type of liquid crystal device in which reset potentials are applied to respective data lines before image signals are supplied to the data lines, and also in a type of a projection display that employs the same.
To solve the problem described above, according to the present invention, there is provided a liquid crystal device equipped with: a pair of substrates with liquid crystal sealed in therebetween; a pixel section which is mounted on one of the substrates and is composed of a plurality of data lines to which image signals are supplied, a plurality of scanning lines which cross the plurality of scanning lines and to which scanning signals are supplied, a first switching element connected to the respective data lines and scanning lines, and a pixel electrode connected to the first switching element; and a reset driving circuit provided with a second switching element for supplying a reset signal, which has been supplied to a reset signal conductor, to the data lines prior to the supply of an image signal in the area around the pixel section, and a capacitor which is connected to the second switching element and which stores electric charges, the pair of substrates is a liquid crystal device bonded to each other by a sealing layer formed in the area outside the pixel section. The capacitor is disposed in an area where the sealing layer is formed and has a pair of electrodes, namely, a first electrode to which a predetermined potential is supplied and a second electrode which is electrically connected to the reset signal line and is disposed opposite to the first electrode via an insulating film.
In the liquid crystal device in accordance with the present invention, the first substrate is provided with a capacitor for increasing the time constant for the reset signal line, the capacitor being formed in the area where the sealing layer is formed. Thus, in the liquid crystal device according to the present invention, since the time constant for the reset signal line can be sufficiently larger than that of the time constant for the data lines, no signal sneaks into other data lines via the reset signal line when the reset potentials are applied to the respective data lines. Therefore, even in the case of a type of a liquid crystal device in which the reset potentials are applied to the respective data lines before image signals are supplied to the data lines, the lateral cross talk or the like due to circulation of signals does not occur, making it possible to improve the quality of display. Moreover, the capacitor for increasing the time constant for the reset signal line is conventionally formed in the area where the sealing layer is formed and where there used to be a dead space in the past; hence, even if a capacitor with a larger capacitance is formed, it would not be necessary to increase the size of the liquid crystal device substrate, or it would not be required to reduce the area where liquid crystal is sealed in and the pixel section is included.
According to the invention, the reset signal line may be composed of a plurality of wiring layers arranged in parallel and reset signals having different potentials of the plurality of wiring layers may be supplied. In this case, the second electrode will be electrically connected only to predetermined wiring layers (reset signal line) by electrically connecting the second electrode to the wiring layers via a contact hole.
In the invention, it is preferable that the first electrode is composed of a plurality of electrode layers extended from the potentiostatic line toward the reset signal line, while the second electrode is composed of a plurality of electrode layers extended from the reset signal line toward the potentiostatic line. In other words, it is desirable from the viewpoint of layout to arrange the reset signal conductor and the potentiostatic line in parallel to the periphery of the pixel section; therefore, the area between the reset signal line and the potentiostatic line should be provided as the area where the sealing layer is formed so that a capacitor is fabricated therein by extending the electrode layers from both the reset signal line side and the potentiostatic line side.
Preferably, in the invention, the first electrode and the second electrode are both constituted by electrode layers having various interlayers formed at the same time when one of the scanning lines, the data lines, and the source-drain region of the thin film transistor is formed. By so doing, the capacitor can be formed without adding to the number of manufacturing steps.
For example, there is a case where one of the first and second electrodes is composed of an electrode layer formed at the same time when the scanning lines are formed, while the other electrode is composed of an electrode layer formed at the same time when the data lines are formed. In this case, a dielectric film of the capacitor will be an insulating film that is formed at the same time when the interlayer insulating film of the thin film transistor is formed in the portion where the first electrode and the second electrode overlap each other.
Further there is another case where one of the first and second electrodes is composed of an electrode layer formed at the same time when the scanning lines are formed, while the other electrode is composed of an electrode layer formed at the same time when a source-drain region of the thin film transistor is formed. In this case, a dielectric film of the capacitor will be an insulating film that is formed at the same time when a gate insulating film of the thin film transistor is formed in the portion where the first electrode and the second electrode overlap each other. This composition enables the capacitor to have a larger capacitance (time constant of the reset signal line) because the gate insulating film, which is thinner than the interlayer insulating film, is used as the dielectric film.
Furthermore, there is another case where one of the first and second electrodes is composed of an electrode layer formed at the same time when the scanning lines are formed, while the other electrode is composed of two electrode layers comprised of an electrode layer formed at the same time when the data lines are formed and an electrode layer formed at the same time when a source-drain region of the thin film transistor is formed. In this case, the capacitor includes a first capacitor that employs, as a dielectric film thereof, an insulating film that is formed at the same time when an interlayer insulating film of the thin film transistor is formed in the portion where an electrode layer formed at the same time when the scanning lines are formed overlaps an electrode layer formed at the same time when the data lines are formed, and a second capacitor employs, as a dielectric film thereof, an insulating film that is formed at the same time when a gate insulating film of the thin film transistor is formed in the portion where an electrode layer formed at the same time when the scanning lines are formed overlaps an electrode layer formed at the same time when a source-drain region of the thin film transistor is formed. This composition makes it possible to electrically connect in parallel the first capacitor using an interlayer insulating film as a dielectric film thereof and the second capacitor using a gate insulating film, which is thinner than the interlayer insulating film, as a dielectric film thereof; hence, the capacitance of the capacitors (time constant of the reset signal conductor) can be further increased.
This invention does not have a driving circuit on a liquid crystal device substrate and it can be applied not only to a type of liquid crystal device adapted to receive scanning signals and image signals from outside but also to a liquid crystal device employing a liquid crystal device substrate made integral with a driving circuit that includes a data side driving circuit for supplying the image signals to the data lines or a scanning side driving circuit for supplying scanning signals via the scanning lines.
An electro-optical device in accordance with the present invention has a pixel area including pixel electrodes arranged in a matrix pattern and a first switching element connected to the pixel electrodes, and a driving circuit for driving the pixels disposed around the pixel area on a first substrate, the substrate being bonded to a second substrate by a sealing layer formed in an area outside the pixel section;
wherein a capacitor composed of a first electrode connected to signal lines from the driving circuit and a second electrode formed to be opposed to the first electrode via an insulating film is formed in the area where the sealing layer is formed.
Thus, according to the invention, the capacitor can be formed in the area where sealant is formed to add a capacitance to the signal lines connected to the driving circuit; hence, the time constant of the signal lines can be increased and the area of the sealing layer which used to be a dead space can be effectively utilized, thus obviating the need of increasing the size of the electro-optical device.
According to the invention, there is provided an electro-optical device that has a plurality of data lines to which image signals are supplied, a plurality of scanning lines to which scanning signals are supplied, a first switching element connected to each of the data lines and the scanning lines, and a pixel electrode connected to the first switching element mounted on a first substrate, and further includes a reset driving circuit equipped with a second switching element for supplying reset signals, which have been supplied to a reset signal line prior to the period during which image signals are supplied to the data lines,and a capacitor connected to the reset signal line.
According to the invention, even if a reset driving circuit for resetting at the same time all the image signals applied to the pixel electrode is provided, the total wiring capacitance of the reset signal line or the total ON resistance of the second switching means increases to enable the reset signals to be written to all the data lines. As a result, the potentials of the respective data lines will be all the desired potentials, so that image signals are written under an ideal condition. This prevents uneven contrast.
The electro-optical device such as the liquid crystal device in accordance with the present invention can be employed, for example, as an electronic equipment such as a projection display device that has a light source section and a projecting means for projecting a light beam, which has been emitted from the light source section and modulated by the liquid crystal device onto a projection surface such as a screen.